Hydrogen generating system and hydrodehalogenation method

ABSTRACT

The invention concerns a hydrogen generating system, characterized in that it combines a water-corrodible metal, an inorganic material, said material having a specific surface capable of fixing the oxide and/or hydroxide form(s) of said metals generated during corrosion. The invention also concerns a method for generating hydrogen and its uses, in particular in a hydrodehalogenation process of halogenated organic compounds present in aqueous media to be purified.

The present invention relates principally to a hydrogen generatingsystem. It also relates to a method of hydrogen generation andapplications thereof, particularly in a method of hydrodehalogenation ofhalogenated organic compounds present in aqueous media to be purified.

The present invention relates more particularly to the catalyticdegradation of pollutants, more particularly of volatile halogenatedorganic compounds (VHOC) such as perchloroethylene (PCE), vinyl chloride(VC), dichloroethane (DCE), dichloroethylene, chloroform, carbontetrachloride and trichloroethylene (TCE), . . . .

This type of degradation is usually carried out by the use of a methodwhich generally involves a reduction of these pollutants with the aid ofhydrogen, in the presence of a so-called hydrodehalogenation catalystgenerally based on palladium fixed on an inorganic carrier. Moreprecisely, the method proceeds with a reducing dehalogenation of thehalogenated components in the course of which halogen is extracted fromthe molecule in the form of a free halogenated ion in aqueous solutionand replaced on the molecule by a hydrogen ion. This type of reactiontherefore requires a source of electrons.

Two alternatives are available nowadays for carrying out this type ofreaction.

According to a first variant, a zero valence metal is used, preferablyiron, as the source of electrons and source of metal for thedehalogenation. This approach has the advantage that it is relativelyinexpensive but on the other hand it has the drawbacks that it is notvery quick and it does not suit all the VHOC and particularly vinylchloride.

The second alternative for its part involves the use of a catalyticsystem which is more elaborate and therefore more costly, which moreoverat the same time requires the use of a continuous source of hydrogen.This hydrogen is generally introduced in the form of gaseous hydrogen orgenerated in situ with the aid of a complex such as hydrazine orborohydrides.

The object of the present invention more precisely is to propose a thirdalternative to the two alternatives referred to previously, this thirdalternative being based more particularly on the use of an originalhydrogen generating system.

Unexpectedly, the inventors have demonstrated that it is possible toprovide satisfaction simultaneously in terms of cost and of kineticswith the aid of a hydrogen generating system which involves inparticular the use of a zero valence metal such as iron as the source ofelectrons.

More precisely, the present invention relates primarily to a hydrogengenerating system, characterised in that it combines a water-corrodiblemetal with an inorganic material, the said material having a specificsurface area which favours the fixing of the oxide and/or hydroxideform(s) of the said metal generated during corrosion thereof.

Advantageously, the combination of an inorganic material with the metalconsidered as a source of electrons in fact permits a significantincrease in the quantity of hydrogen generated by comparison with aconventional method, that is to say one using only the zero valencemetal.

In so far as the zero valence metal is concerned, it has an oxidationreduction potential which is sufficiently negative to be able to reducethe water.

As representative examples of metals suitable for the invention, mentionmay be made in particular of steel, iron, zinc, aluminium, tin, bismuth,cobalt and nickel.

Zero valence iron is preferred, this being of particular interestbearing in mind its low cost.

With regard to the inorganic material, this is preferably chosen fromamongst the metal oxides, mixed or unmixed, in so far as they are ofcourse inert in the reaction conditions.

As representative examples of these oxides, mention may be made inparticular of aluminas, silicas, the oxides of zirconium, cerium,titanium and iron, and zeolites.

The inorganic materials may be used in different forms: powder, shapedproducts such as granulates (for example cylinders or beads), pellets,monoliths (blocks in honeycomb shape) which are obtained by extrusion,moulding, compaction or any other type of known method. In practice, onthe industrial scale these are in the form of granulates, beads ormonoliths which have the most advantages not only in terms ofeffectiveness but also in terms of the convenience of using them.

The inorganic material preferably has a specific surface area greaterthan that of the zero valence metal.

The inorganic materials generally have a specific surface area greaterby at least a factor of 100, and preferably of 10⁴, than that of themetal, and this factor can rise to a value of 10⁶.

According to a preferred embodiment of the invention, this is asynthetic or natural zeolite.

Zeolite is understood to mean a crystallised tectosilicate of natural orsynthetic origin of which the crystals result from the three-dimensionalassembly of tetrahedral units of SiO₄ and TO₄, T representing atrivalent element such as aluminium, gallium, boron and iron, preferablyaluminium. Zeolites of the aluminosilicate type are the most common.

Amongst the zeolites it is possible to use natural zeolites such as forexample offretite, clinoptilotite, erionite, chabazite and philipsite.

Synthetic zeolites are also suitable.

As examples of synthetic zeolites with a one-dimensional lattice,mention may be made inter alia of zeolite ZSM-4, zeolite ZSM-12, zeoliteZSM-22, zeolite ZSM-23 and zeolite ZSM-48.

As examples of zeolites with a two-dimensional lattice which arepreferably used, mention may be made of zeolite beta, mordenite andferrierite.

Use is preferably made of synthetic zeolites and more particularly thosewhich are in the following forms:

-   -   mazzite with a molar ratio Si/Al of 3.4,    -   zeolite L with a molar ratio Si/Al of 1.5 to 3.5,    -   mordenite with a molar ratio Si/Al of 5 to 15,    -   ferrierite with a molar ratio Si/Al of 3 to 10,    -   offretite with a molar ration Si/Al of 4 to 8.5,    -   zeolites beta with a molar ratio Si/Al of 15 to 25,    -   zeolites Y, in particular the zeolites obtained after        dealumination treatment (for example hydrotreatment, washing        with the aid of hydrochloric acid or treatment with SiCl₄), more        particularly zeolites US-Y with a molar ratio Si/Al greater than        3, preferably between 6 and 60,    -   zeolite X of the faujasite type with a molar ratio Si/Al of 0.7        to 1.5,    -   zeolites ZSM-5 or aluminium silicate with a molar ratio Si/Al of        10 to 2000, and    -   zeolite ZSM-11 with a molar ratio of 5 to 30.

The inorganic material is preferably a zeolite having a specific surfacearea greater than 10 m²/g.

The two zeolites described in the following examples have proved to beof particular interest within the scope of the present invention.

In fact, without wishing to be tied to a specific mechanisticexplanation, it appears that the inorganic material acts as a specificcarrier with regard to metal oxides and/or hydroxides generated duringthe oxidation of the zero valence metal.

In fact, these hydroxides are generated automatically during thereaction of water on the metal according to the following scheme:i. M⁰M^(n+)+ne⁻ii. 2H₂O+2e⁻H₂+2OH⁻

${{iii}.\mspace{14mu} M} + {{n\left( {H_{2}O} \right)}\mspace{20mu}{M({OH})}_{n}} + {\frac{n}{2}H_{2}}$

Thus by becoming fixed preferably on the inorganic material and not onthe surface of the metal in the zero valence state, the generated metalhydroxides significantly limit the deactivation of the latter.

Thus it is that in the particular case where the generation of hydrogenis carried out by combining 3 g of iron with 15 g of water, and in thepresence of a carrier such as a zeolite, the production of hydrogengenerated over a period of 24 hours proves to be 250 times greater thanthat generated under the same operating conditions but in the absence ofthe said carrier.

According to an advantageous embodiment of the invention, the zerovalence metal and the inorganic material are combined in a proportion of0.5 to 40%, preferably 1 to 20%, by weight of the said material relativeto the weight of the metal.

This adjustment between the two compounds is also of course a functionof the specific surface area of the inorganic material. It will beunderstood that the necessary quantity of inorganic material isinversely proportional to its specific surface area.

According to a preferred variant, the zero valence metal is not carriedby the inorganic material which is combined with it. It is equallypossible to envisage that the inorganic material might also serve as acarrier for another metal capable of intervening by way of catalyst fora reaction following or accompanying the hydrogen generation.

The hydrogen generating system claimed is of particular interest in thefield of treatment of effluents for example for the reduction of theVHOC and/or nitrates, in the field of reduction of nitrated compounds,particularly aromatic ones. In fact it is generally possible to envisageexploitation of this system for any application necessitating acontinuous supply of hydrogen.

The present invention also relates to a method of hydrogen generation byreduction of water with the aid of a suitable metal, characterised inthat the said reduction is carried out within an aqueous medium in thepresence of a sufficient quantity of an inorganic material of which thespecific surface area favours the deposition of the metal oxide and/orhydroxide form(s) generated in the course of the said reduction.

Naturally, the inorganic material and the metal conform to thedefinitions presented above within the scope of the claimed system.

In the claimed method, the inorganic material and the metal can bedirectly introduced into the aqueous medium to be treated, and theaggregate is then agitated in such a way as to optimise the conditionsof hydrogen generation.

Another variant of the method may consist of causing the aqueous mediumto be treated to circulate through a fixed bed comprising at least thesaid metal and the inorganic material.

The present invention also relates to the use in a method of hydrogengeneration by reduction of water with the aid of a suitable metal, of aninorganic material for the purpose of fixing the oxide and/or hydroxideform(s) of the said metal generated during the reduction.

The present invention also relates to the application of a system forhydrogen generation such as is defined above for the hydrodehalogenationof volatile halogenated organic compounds within an aqueous medium.

More precisely, it proposes a method of hydrodehalogenation of volatilehalogenated organic compounds present in an aqueous medium,characterised in that it uses hydrogen generation by a hydrogengenerating system according to the invention and catalytichydrodehalogenation of the volatile halogenated organic compounds withthe aid of the hydrogen thus formed and a suitable carried catalyst.

Unexpectedly, the inventors in fact noted that the beneficial effect ofthe presence of an inorganic material for the generation of hydrogencould also be exploited effectively for the hydrodehalogenation. Thefact that the hydrodehalogenation catalyst is placed in an environmenthighly enriched with hydrogen permits the kinetics ofhydrodehalogenation to be increased considerably. This advantageousaspect of the claimed method is demonstrated in particular by theexamples presented below.

The inorganic material present in the system is advantageously used in aquantity such that its developed (that is to say, total) surface isgreater, preferably by at least a factor of 5, than the developedsurface of the inorganic material constituting the carrier for thehydrodehalogenation catalyst. Such a developed surface advantageouslypermits the catalytic yields of the metal forming thehydrodehalogenation catalyst to be preserved over time.

With regard to the dehydrodehalogenation catalyst, it generallycomprises as the metal a metal chosen from amongst palladium, nickel,ruthenium, platinum and/or rhodium. It is preferably palladium.

This metal is likewise carried on an inorganic material. This inorganicmaterial may be chosen from amongst those defined previously. It ispreferably an alumina or a zeolite.

Palladium fixed on alumina may be mentioned as the hydrodehalogenationcatalyst more particularly preferred within the scope of the invention.

It may be envisaged that the carrier which has the hydrodehalogenationcatalyst fixed on its surface simultaneously ensures the functioning ofthe inorganic material involved in the hydrogen generation reaction.According to this variant, it proves possible to reduce significantlythe quantity of metal which forms the carried catalyst. The metal whichforms the carried catalyst can then be advantageously used in aproportion of 10 to 150 mmole/kmole of the zero valence metal.

Thus in the particular case where the zero valence metal is iron and thecarried catalyst is palladium/alumina, the hydrodehalogenation methodcan be advantageously carried out with a mass ratio palladium/iron ofless than 100 mg of palladium/kg of iron in comparison with 500-5000 mgPd/kg Fe for conventional methods.

The claimed method may be applied to the reduction of all the organiccompounds represented by the families of chlorinated solvents such astrichloroethylene, the chlorinated aromatics such as chlorobenzene, thechlorophenols or also products for the protection of plants such asLindane™, Dinoterbone™ and nitro compounds.

Thus the claimed dehalogenation method may be applied to thepurification of ground waters within a range of temperatures which mayvary from 4 to 35° C. It may in particular be carried out within areactor.

According to a preferred variant of the invention, the hydrogengenerating system and the hydrodehalogenation catalyst are separatedwithin the reactor. The zero valence metal and the inorganic materialare placed in the lower part of the reactor, at the level of which theliquid medium to be treated is introduced. The hydrogen generated inthis lower part then travels towards the upper part of the reactor wherethe hydrodehalogenation catalyst is placed.

According to this arrangement, the metal oxides and/or hydroxides formedduring the hydrogen generation are preferably deposited on the inorganicmaterial present in the lower level of the reactor and not on thecarried hydrodehalogenation catalyst. In this way, the activity of thehydrodehalogenation catalyst is optimised and its performances aremaintained.

The examples which appear below are presented by way of example and donot limit the scope of the invention.

The carriers tested in the following examples are two zeolites US-Y (4%of Na₂O₃ with Si/Al of 2.5) (sold by Engelhard) known as zeolite A and azeolite HY-CBV 400 (2.5% Na₂O₃ and Si/Al of 1.5 and a specific surfacearea of 50 m²/g) (sold by Zéolyst International) known as zeolite B.

EXAMPLE 1

15 g of water, 3 g of iron and 300 mg of a zeolite A or B are mixed.After agitation over a period of 24 hours it is noted that, in each ofthe tests, the quantity of hydrogen generated is 250 times greater thanthe quantity of hydrogen generated in a control test, that is to say inthe absence of zeolite.

It is in fact 16.5 ml/kg water/hour as against 0.067 ml/kg water/hourfor the control test.

EXAMPLE 2

1.5 mg of trichloroethylene is introduced in the form of a solution atapproximately 100 ppm into 15 g of water. Also introduced into thissolution are 3 g of iron and 30 mg of a Pd/Al₂O₃ catalyst. Afteragitation for 18 hours and 30 minutes, a transformation rate of 30% ofthe trichloroethylene is achieved as against a transformation rate ofonly less than 5% for this same system in the absence of the saidcatalyst.

In a variant of this test, the reaction is carried out in the presenceof 300 mg of alumina (alumina CBL sold by Procatalyse).

Under these conditions, a transformation rate of 100% is achieved in 18hours and 30 minutes. The only reaction product observed is ethane.

1. Hydrogen generating system comprising a water-corrodible metal, anaqueous medium, and an inorganic material having a specific surfacearea, and on which is fixed the oxide and/or hydroxide form(s) of themetal generated during corrosion thereof, wherein the inorganic materialhas a specific surface area greater by at least a factor of 100 thanthat of the metal, and wherein the water-corrodible metal is present inthe aqueous medium and is not carried by the inorganic material, whereinthe inorganic material is a synthetic or natural zeolite.
 2. System asclaimed in claim 1, wherein the metal has a negative oxidation reductionpotential.
 3. System as claimed in claim 1, wherein the metal isselected from the group consisting of steel, iron, zinc, aluminum, tin,bismuth, cobalt and nickel.
 4. System as claimed in claim 1, whereinsaid metal is iron.
 5. System as claimed in claim 1, wherein theinorganic material is a zeolite having a specific surface area greaterthan 10 m²/g.
 6. System as claimed in claim 1, wherein the metal and theinorganic material are combined in a proportion of 0.5 to 40% by weightof the material relative to the weight of the metal.
 7. Method ofgenerating hydrogen, comprising producing a continuous supply ofhydrogen from the hydrogen generating system as defined in claim
 1. 8.Method of conducting a hydrodehalogenation reaction, comprisingconducting a hydrodehalogenation reaction with the hydrogen generatingsystem as defined in claim 1, wherein volatile halogenated organiccompounds are present in the aqueous medium.
 9. Method ofhydrodehalogenation of volatile halogenated organic compounds present inan aqueous medium, comprising conducting hydrogen generation with thehydrogen generating system as claimed in claim 1 and catalytichydrodehalogenation of the volatile halogenated organic compounds in thepresence of the hydrogen thus formed and a catalyst.
 10. Method asclaimed in claim 9, wherein the catalyst comprises a metal selected fromthe group consisting of palladium, nickel, ruthenium, platinum andrhodium.
 11. Method as claimed in claim 10, wherein the metal is fixedon a second inorganic material.
 12. Method as claimed in claim 11,wherein the inorganic material present in the hydrogen generating systemis used in a quantity such that its developed surface is greater thanthe developed surface of the second inorganic material.
 13. Method asclaimed in claim 9, wherein the catalyst is palladium carried on aluminaand the metal of the hydrogen generating system is iron.
 14. Method asclaimed in claim 13, wherein the mass ratio palladium/iron is less than100 mg of palladium/kg of iron.
 15. Method as claimed in claim 9,wherein the aqueous medium is treated within a reactor in which thehydrogen generating system is separated from the catalyst.
 16. Ahydrogen generating system comprising an aqueous medium containing awater-corrodible metal and an inorganic material, said inorganicmaterial having a surface area which fixes thereon the oxide and/orhydroxide form(s) of the metal generated during corrosion thereof,wherein the inorganic material has a specific surface area greater by atleast a factor of 100 than that of the metal, and wherein thewater-corrodible metal is present in the aqueous medium and is notcarried by the inorganic material, wherein the inorganic material is asynthetic or natural zeolite.
 17. A hydrogen generating system accordingto claim 1, wherein the oxide and/or hydroxide form(s) of the metalgenerated during corrosion is fixed directly on the inorganic material.18. A hydrogen generating system according to claim 16, wherein theoxide and/or hydroxide form(s) of the metal generated during corrosionis fixed directly on the inorganic material.
 19. A hydrogen generatingsystem according to claim 16, wherein the inorganic material is azeolite having a specific surface area greater than 10 m2/g.